Acute Respiratory Failure

Question 1

Hypoxemia

Answer 1

Defined as low PO2

Causes include:

1) V/Q mismatch
2) R to L shunt
3) Hypoventilation
4) Low inspired O2 content (altitudes)
5) Diffusion impairment

Question 2

History and physical with hypoxemia

Answer 2

Findings depend on etiology. Low HbO2 saturation, cyanosis, tachypnea, SOB, pleuritic chest pain and AMS may be seen

Question 3

Dx of hypoxemia

Answer 3

1) Pulse ox: demonstrates low HbO2 sat
2) CXR: evaluates for an infiltrative process (pneumonia), atelectasis, a large pleural effusion, or pneumothorax and to assess for ARDS
3) ABG: Calculate the alveolar-arterial (A-a) oxygen gradient.
4) An increased A-a gradient suggests shunt, V/Q mismatch or diffusion impairment.

Question 4

How do you calculate A-a gradient?

Answer 4

X) P atm - 47
Y) PaCO2 divided by 0.8

A-a gradient = (XFiO2 - Y) - PaO2

Question 5

Tx of hypoxemia

Answer 5

1) Address underlying etiology
2) Administer O2 before initiating evaluation
3) Increase oxygenation parameters if the patient is on mechanical ventilation
4) In hypercapnic patients, increase ventilation to increase CO2 exchange.

Question 6

Approach to determining mechanism of hypoxia

Answer 6

1) Is PaCO2 increased?

A) Yes = hypoventilation
B) Is PAO2-PaO2 increased?

B1) Yes = hypoventilation plus another mechanism
B2) No = Hypoventilation alone. Either reduced respiratory drive or NM disease

2) Is PaCO2 increased?

A) No
B) Is PAO2 - PaO2 increased?
B1) No = reduced inspired PO2 from either high altitude or low FiO2. STOP

B2) Yes. Keep going.
C) Is low PO2 correctible with O2?

C1) Yes - this is V/Q mismatch either from airway disease (asthma, COPD) or interstitial lung disease or alveolar disease or pulmonary vascular disease

C2) No - This is a shunt. Either from alveolar collapse (asthma, COPD) or intra-alveolar filling (pneumonia, pulm edema) or intracardiac shunt or vascular shunt within lungs

Question 7

Mechanical ventilator parameters affecting oxygenation and ventilation

Answer 7

Increased oxygenation

1) Increase FiO2
2) Increase PEEP

Increased ventilation

1) Increase RR
2) Increase TV

Question 8

ARDS

Answer 8

Respiratory failure with refractory hypoxemia, low lung compliance and non-cardiogenic pulmonary edema with a PaO2/FiO2 ratio less than 200.

Pathogenesis is thought to be dependent on endothelial injury. Common triggers include sepsis, pneumonia, aspiration, multiple blood transfusions, inhaled/ingested toxins, and trauma

overall mortality is 30-40%

Question 9

History and physical for ARDS

Answer 9

Presents with acute-onset (12-48h) tachypnea, dyspnea, and tachycardia with or without fever, cyanosis, labored breathing, diffuse high-pitched rales, and hypoxemia in the setting of one of the systemic inflammatory causes or exposure.

Phase I (acute injury) - Normal physical, possible respiratory alkalosis

Phase 2 (6-48h) - hyperventilation, hypocapnia, widening A-a gradient

Phase 3 - acute respiratory failure, tachypnea, dyspnea, lower lung compliance, scattered rales, diffuse chest opacity on CXR

Phase 4 - Severe hypoxemia unresponsive to therapy. Increased intrapulmonary shunting. Metabolic and respiratory acidosis.

Question 10

Dx of ARDS

Answer 10

it’s not hARDS to diagnose ARDS

Acute onset
Ratio (PaO2/FiO2) less than 200
Diffuse infiltration (bilateral pulm infiltrates on CXR)
Swan-ganz wedge pressure less than 18

Criteria demand no evidence of cardiac dysfunction which is why the swan-ganz thing is mentioned. Could also just have no evidence of LA pressure issues.

Question 11

Tx of ARDS

Answer 11

1) Mechanical ventilation with low tidal volumes to minimize ventilator-induced lung injury
2) Treat underlying disease and maintain adequate perfusion to prevent end-organ damage
3) Use PEEP to recruit collapsed alveoli and titrate PEEP and FiO2 to achieve adequate oxygenation
4) Goal oxygenation is PaO2 above 60 or SaO2 above 90 on FiO2 less than 0.6
5) Slowly wean patients from ventilation and follow with extubation trials

Question 12

Criteria for extubation from mechanical ventilation

Answer 12

Patients who meet the criteria are given a weaning piece (T piece) trial to determine if they are ready

1) Pulmonary mechanics

VC over 10-15
Resting minute vent (TV x RR) over 10
Spontaneous RR less than 33
Lung compliance over 100
Negative inspiratory force less than -25

2) Oxygenation

A-a gradient less than 300-500
Shunt fraction less than 15%
PO2 (on 40% FiO2) greater than 70
PCO2 less than 45